Not being especially enamored with the looks of most Rugers, I selected the GP100 because the Ruger revolvers do have a reputation for being sturdy. That, and also because I think the aesthetics of the Ruger semi's leaves a lot to be desired.
In a strictly caliber sense, the .357 Magnum is in a whole different category than the vernable .45 ACP. The .357 is much more versatile and can serve several roles such as target shooting, small game, medium game and self-defense. It's about as close to the "do-all" handgun as you can get.